1. Field of the Invention
The present invention generally relates to a display apparatus for displaying, for example, a television image. Moreover, the present invention relates to a method of illuminating a miniaturized light valve with light emitted from a light source, and to a display apparatus in which a light valve modulates the illumination light and then emits the modulated light to form an image.
2. Description of the Related Art
With a conventional projection type display apparatus, light is emitted from a light source and is incident on a light valve and the light valve modulates the incident light in accordance with an image to produce ON-light representative of an image dot. Then, the light valve emits the ON-light to a projection means, which projects the ON-light onto a screen or the like. The illumination light incident on the light valve illuminates the entire surface of the light valve to produce a desired uniform profile of distribution of illuminance across the entire surface of the light valve. In order to obtain a uniform profile of distribution of illuminance, for example, a beam of illumination light emitted from a light source is first directed to transmit through a light integrator in the form of a fly' eye lens sheet or an optical element of columnar structure. This allows the beam of illumination light to have a cross section analogous to the shape of the light valve before forming an image on the light valve.
ON-light emitted from a light valve is, for example, directed to transmit through a projection means to form an image on, for example, a screen. The image formed on the screen is diffused into the eyes of views. Illumination light incident on the light valve and light emitted from the light valve but not projected (referred to as “OFF-light” hereinafter) are unwanted light, i.e., stray light. If such stray light enters the ON-light, the stray light makes the image blurred. For this reason, a means is provided that allows only the ON-light to pass through. If the light valve takes the form of a micro mirror device, then ON-light and OFF-light travel in different paths and therefore a total reflection prism is used in which light is transmitted or not transmitted depending on the direction of travel of light (U.S. Pat. No. 5,552,922, FIG. 1). If the light valve takes the form of a transmission type liquid crystal element or reflection type liquid crystal element, then the ON-light and OFF-light travel in different paths and therefore a polarizing plate or a polarization split prism is employed, thereby allowing only the ON-light to be transmitted onto a projection means (Japanese Patent Application Laid-Open No. 11-38365, pages 5-6, FIG. 1).
When a color image such as television image is to be displayed, one or a plurality of light valves, for example, are used to form images of respective primary colors and combine the respective color images into a full color image. The use of a prism for combining light of the respective colors results in a large overall apparatus size (Japanese Patent Application Laid-Open No. 11-38365, pages 5-6, FIG. 1). For a system that does not require a color-combining prism, illumination light of three primary colors (red, green, and blue) illuminate a sheet of light valve in sequence (Japanese Patent Application Laid-Open No. 8-51633, pages 3-4, FIG. 4). In this case, the apparatus may be miniaturized without difficulty. For micro mirror elements and reflection type liquid crystal elements, display elements operate fast enough in switching between ON-light and OFF-light as compared to the frame time of an image to be displayed, so that the projected light of three primary colors can be switched from one color to another during one image frame and therefore images of the respective colors are sequentially formed in registration.
Another way of combining images of primary colors is implemented by an apparatus in which a single light valve is employed and filters of primary colors are disposed in such a way that a filter of one color is in series with a corresponding path of illumination light and dots of different colors are formed on a pixel-by-pixel basis of the light valve (U.S. Pat. No. 6,254,237, FIGS. 1 and 3). Some the conventional apparatus employ micro mirror elements but do not use a total reflection prism for separating ON-light from the rest (Japanese Patent Laid-Open No. 2002-268010, Page 4, FIG. 1). Further, some of the reflection type liquid crystal elements use a plate-like reflection type polarization means in place of a polarization split prism (U.S. Patent No. 2003-121922, FIG. 1 and FIG. 4).
Yet another conventional apparatus is of a laser scanning type in which an image signal intensity-modulates light emitted from a laser light source and a scanning means (galvanometer and mirror on page 209-223, FIGS. 5-61 in “Practical Laser Technique”, by Toshimitsu Hirai, published by Kyoritsu Shuppan) causes the modulated light to scan on an object (e.g., screen) to form an image.
If conventional display apparatus are to be miniaturized in overall size or reduced in thickness, the apparatus suffer from the problems that the entire surface of a light valve can not be illuminated uniformly and noise enters the image to be displayed. A description will be given of causes of such problems as follows.
A display apparatus that uses a reflection light valve requires a light path for illumination light and a means for removing unwanted light, the light path and the means being provided near the light valve. The illumination light illuminates the light valve uniformly. The means for removing unwanted light allows ON-light to be viewed without noise. In order to implement these functions, glass prisms such as a total reflection prism (U.S. Pat. No. 5,552,922) and a polarization split prism (Japanese Patent Laid-Open No. 11-38365) are employed. These glass prisms are useful for telecentric illumination. In order to prevent noise from entering ON-light, it is necessary to prevent stray light from entering the path of the ON-light. For this purpose, a prism should have a volume large enough for enclosing the light path and thus it is difficult to implement a miniaturized display and a thin type display. A glass prism is made of a homogeneous glass material and processed through polishing and bonding, thus being relatively expensive and heavy. A projection type display incorporates a glass prism disposed between a light valve and a projection means (U.S. Pat. No. 5,552,922, FIG. 1 and Japanese Patent Application No. 11-38365, pages 5-6, FIG. 1). Thus, the back focal length of the projection means is long, making it difficult to design a projection means that is capable of projecting an image with a wide field angle while also maintaining good optical characteristics such as chromatic aberration. This is a barrier to a miniaturized display apparatus having a short projection distance. A display apparatus incorporating a plate-like polarization splitting means in place of a polarization split prism has an advantage of having a small weight as compared to apparatus incorporating prisms (U.S. Pat. No. 6,447,120, FIG. 1). However, because a medium that forms a light path from a light valve to a projection means is air except for the polarization splitting means, the effective back focal length of the projection means cannot be short. A conventional display apparatus according to Japanese Patent Application Laid-Open No. 2002-268010 (FIGS. 1 and 3) incorporates a lens and a mirror in place of a total reflection type prism, and suffers the problem that it is difficult to illuminate a light valve with a required uniform profile of distribution of illuminance.
A display (U.S. Pat. No. 6,254,237) that incorporates a transmission type light valve also requires an optical means such as a fly's eye lens for illuminating a light valve and a path for the light, and is therefore difficult to implement a further miniaturized display apparatus.
With a laser scanning type display, it is required that for displaying individual pixels properly, the image of a light source should be sufficiently small and the light source is sufficiently bright. Such a light source can only be implemented by a large size, expensive laser oscillator.
Image signals have a variety of aspect ratios. For example, a television signal has an aspect of either 16:9 or 4:3. A conventional light integrator is configured to emit illumination light having a predetermined cross-section and therefore a light valve is illuminated with a predetermined profile of distribution of illuminance. If an image to be displayed has an aspect ratio different from the shape of a light valve, some pixel areas on the light valve available for image formation are illuminated by the illumination light but not effectively used to display any image. In this case, such a portion of the illumination light not effectively used is OFF-light and represents about 20% in a total illuminated area, resulting in poor efficiency of light utilization.